I'd like to clarify exactly what you mean. You are saying that the enemy has an off-tank champion who is fed. And you are asking if it is better to have YOUR (not fed) off-tanks rush to THEIR AD carry, and hope that it will force the enemies fed off-tank to protect his carry and not dive to your allied carry. Correct?
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RovingBladeJul 22 '12 at 3:44

As fed AD carry against a fed Offtank/Bruiser you should get an GA asap and get your team to protect you. You can wear down the enemy Offtank (kite him) and then kill the enemy AD since you are stronger, while if you get killed by the enemy Offtank, your team will loose the fight for sure (since they lack sustained DMG)

I don't think that the second plan will work, as most Tanky-DPS champions build in a way that the could solo an AD Carry (if they get next to them). Imagine allowing a fed jax to reach your carry... the carry will melt, and then jax will clean up the rest of your team afterwards.

If your AD Carry is fed, you have an advantage and you need to protect that advantage rather than dive for the enemies non-threatening, non-fed AD Carry.

In team fights where your team is very farmed, the proper way to fight is to focus the same -closest- enemy, no matter who it is, and move on to the next closest. This is especially true for a farmed carry, as this ensures safety.